Photoelectric exposure meter



June '19, 1945. l., RlszDoRFER PHUTOELECTRIC XPO'SURE 'METER Invzz ton Lszl IlSzdorfer Filed Sept. v23, 1940 l Patented June 19, 1945 --UNITEDSTATE s PATENT OFFICE f l Iszl Rlszdorfer, Budapest I, Hungary; Avestedin the Alien Property Custodian Applicationv september z3, 1940, serialNo. 357,992

' In Hungary August 26, 1939 l sclaimslwiss-za) The magnitude oftheangle of incidence under which the light falls on the photo-cells of theexposure meters of the types known up to now, as well as on those of thesemi-automatic or automatic light control devices of the photo-cell typeem- Dloyed in connection with photographic cameras,

amounts to about 60 to 80 degrees; in the case of some devices thisangle is even greater. This is due to the fact that in case of a smallerimage angle,

the quantity of light transmitted to the cell will not be sumcient tosatisfy practical requirements.

Owing to the useof an image angle of so great magnitude, it is anaverage quantity of light that will reach the photo-cell, so that thephoto-cell will practically never be operated by the light of a denitepoint, but by the light reaching' the photo-.cell from the sky, from theground and from various other objects jointly, i. e. by the mean figureof the quantities of light reachingthe cell from all these differentquarters.

4In practice this gives rise to differentV defects.

Notably, in the case oi' exposure meters of this type of design, orinthe case of photographic cameras equipped with a semi-automatic orcom. pletely automatic device of the .photo-cell type operated by meansof a photo-cell having suchan image angle, the measurement ofillumination eilected'on the basis oi' the average quantity of lightreaching the\cel1,and accordingly the adjustment for perfect exposure:of the photographic cameras equipped with the said devices, will onlybe perfect i! the source'ofxlight (sun or lamp)y is situated behind-theexposure meter,

i. e. behind the person taking'the photograph, that is to say if thetaking of the photographs is effected with so-called flat illumination.If,

however, thetaking of the-photograph or the.

and white emulsicn' substance, this fauit will not -manifestitselfinany-striking manner in the ease of the devices forthe measurement of thenecessary amount of exposure or for the control of light which areoperated on the basis of an average quantity of light of the typesemployed up to now, but it will present itself all the more strikngly'inthe case of colour photography, for which, accordingly, the devices ofthis kind can only lbe used in the case of the so-calied fiat exposureof'the subject. Notably, in the case of side or opposed light itisnecessary, for the purposes of measurement, either to approach thesedevices quite closely to the shadow portions of the object to bephotographed,v which means a great deal of inconvenience and loss oftime, or to act according to a judgment of conditions of illuminationbased on an estimate, which method type which up tonow has beenmeasuring excl'usively the mean iigure of illumination-which device maybe not only an exposure meter but also a semi-automatic or completelyautomatic measurement of the necessary amount Iof exposure takes placein side or opposed light, this measurement oi' thel necessary 'amount'otexposure according to 'average quantity .of light..

and, accordingly, the exposureeiected by means of a camera, equippedwith a device of the kind referred to will be incorrect. This is due tothe fact that 4it is always on the baslsof adjustment v for the parts inshadow that th'e measurement of the necessary amount of exposure andvthe exposure itself, respectively, have to be eieeted. A, Thismeans inpractice that theactual amount or exposure which has to be employed willin the case of side light exceed the figures oi exposurev obtalnedon thebasis of the average quantity ofl nina hr1-about ao wsoamuie casseropposed u light b y 50 to 190% or possibly even bymore.-

' `-Owing to the great latitude otleredby the black 'light controldevice-there is added an apparatus automatically compensating ,the Vsaiddevice,

' thereby enabling that thedevice ShOuld take into accountnot only themean degree of illumination, but also the contrasts of light orcontrasts of illumination of the object-'to be photographed', and alsoits colour values.

Notably, the contrasts of illumination'of th object depend in the iirstplaceon the fact from what' angle, based on the position of thephotographer, the vobject to be photographed is illuminated by thesource oflight from which light falls on it. Notably, if the light fallsonthe object from behind the' photographenthe data of measurement basedon the averagequantity of light do'notdequire any correction. It, onvthe other hand, it is from the side that' the light of thesource oflight, e. g. the light of sun, Ialls on p lthe object, a correction ofthe data will 'already become necessary, whereas if itis from'behin'dvIiQ the object toile photographed that the light falls on the latter,the correction will have vto be even greater.

It is, accordingly, evident, that the greater the intensity of the lightto be dealt with, e. g.

intense sunlight, and the less it is from the front cording to theinvention, this being done in such' a manner thatv the said apparatuswill automatically correct the exposure meter so as to take these pointsof view into account.

The simplest Way to achieve this is by employing a type of apparatus inwhich the operation of the device for the measurement of the necessaryamount of exposure or for light control is influenced in a suitablemanner by another device of the photo-cell type, or possibly without theemployment of a special photo-cell, either by optical or by electrical,or by mechanical or by any other known means.

The energy possessed by these compensating devices of the photo-celltype Will, of course,- only have to be sufficient for suitablycorrecting the energy of the exposure meter proper. Another fundamentalcondition of successful operation is that thedevice according to theinvention for the measurement of the necessary amount of exposure or forlight control will have to be calibrated in such a manner that in thosecases when no correcting effect whatever has to be displayed by thecompensating device, as for instance in case of measuring the necessaryamount of exposure of an object illuminated by entirely fiatillumination, such an exposure meter should indicate iigures exactlyequal to those indicated by another device for the measurement of thenecessary amount of exposure or for light control not equipped with anycompensating apparatus according to the invention.

It is a matter of course that all devices according tothe invention forthe measurement of exposure or for light control can be employed notonly for all kinds of photometric measurements, but also for correctingthe operation of semiautomatic or completely automatic devices of thephoto-cell type employed in connection with photographic cameras, butfor the sake of greater simplicity we shall, in the remaining part ofthis specification, pay no attention to this circumstance and refer tothe devices according to the invention as if they were exposure meters lonly.

The plane of the surface of the compensating cell may form differentangles with the plane of the fundamental cell.

For the sake of greater simplicity the cells.

falling from the source of light on the subsidiary cell is suilicientfor causing thel self-adjustment of the pointer of the instrumentoperated by the main cell to be influenced in such a degree as to ensurethat the' said pointer should always, independently of the position ofthe sourceof light,`

indicate the diaphragm or shutter gure corresponding to perfectexposure. That is to say, whether it is light coming from behind orlight from the side or opposed light that is concerned* which opposedlight may, of course, only be opposed light taken in the photographicsense, i. e. may not be, or may only in exceptional cases be, suchopposed light as is situated within the image angle of the illuminationmeter, or, respectively, as shines also into the lens of thephotographic camera-the exposure meter should always indicate thatdiaphragm or shutter figure which is required for the purposes ofcorrect exposure. In'other words, it should in the case of side lightindicate an amount of exposure in creased by about 30 to 50%, and in thecase of opposed light an amount of exposure increased by 50 to 100%, orpossibly by even more, as compared to the amount of exposure indicatedby an exposure meter of the types employed up to now, to the indicationof which latter 30 to 100% or even more have, exactly for this reason,te be added on the basis of an estimate, in the case of side or opposedlight. The amount of correction will be different in eachcase and willvary from the smallest amount to the maximum, and' therefore it isbetter to speak of limits of correction varying from 1 to 400%. In viewhereof it is necessary that the quantity of energy received from' thesource of light by the subsidiary cell should be all the greater, thegreater the difference by which the diaphragm or shutter figureindicated by the exposure meter not equipped with the apparatusaccording to the invention is smaller than the gure required from thepoint of View of correct exposure.

This purpose can be achieved by ensuring that, differently from what isthe case with the main cell adjusted on the object to be photographed,the image angle of which cell is relatively low, the subsidiary cellshould receive light practically only from the source of light and notfrom the object, although some light may reach it from the latter also.If the image angle of the subsidiary cell is very large, amounting e. g.to degrees, and if at the same time its plane surface extends duringmeasurement in a direction sloping upward, this is already suiiicientfor ensuring that if the light of the sun is approximately opposite toit, which means that the rays are falling on its surface approximatelyperpendicularly, the subsidiary cell should exert the maximum ofcompensating effect. On the other hand, if in the case of side light thelight reaches the said plane slightly from the side, the subsidiary cellwill in that case also exert its compensating eect, but in asubstantially smaller extent, and this is indeed what is necessary,because in the case of photographs taken with side light it is notnecessary to increase the exposure, as compared to the iigures indicatedby the instruments of the photo-cell type employed up to now, by so muchas would be necessary in those cases when the sun more or less faces thephotographer.

'In order to ensure .the perfect operation of the device according V.tothe invention, it is, howevernot only -required that the mutualproporreach the cell from different directions,A in aci .rendering themutual eects of the two cells on each other perfect, as for instance allinstruments, covering plates, etc., being of such a kind as to renderthe compensation perfect in all cases. AIt is easy to take all thesefactorsinto account empirically, and to ensure hereby `that the deviceaccording to the invention should in all cases operate in aperfectmanner;

Calibration is facilitated bythe fact thatit is necessary that in thecase of at lighting, i. e.

in case the source of 4light illuminates the object to be photographedfrom behind the' photographer, and in case there is hardly any -shadowportion on the object to be photographed, the exposure meter accordingto the invention should indicate figures equal to those .indicated bythe exposure meters of the types employed up to now voperating on thebasis of an average quantity of light.

The exposure meter equipped with an automatic compensating deviceaccording to the invention should be amplified bythe employment ofacolour filter arranged in front of the cell or cells, which filter'willin case the photo-cell is less sensitive for the so-calledphotographically dark colours, correct the measurement of light in asuitable manner from the point of view of such colours, in the firstplace o'f green, brown and red colours, situated on the area of theobject to be photographed. The practical arrange- ',ment to be employedmay be of very many kinds, because, as already mentioned above, it-isnot cordance with practical necessity.

Of course, there is nothing to preventpthe quantity of the lightreaching the photo-cell either in case of the main or of the subsidiaryphoto-cell, the mutual angle formed by their may also be varied so as tomount or construct the cells I and 2 so as to be situated in the sameplane, possibly mounted on a common rear electrode plate.

Arrangements are also possible-by means of which the side light also maybe projected in a certain extent on the subsidiary cell 2 by means ofmirrors or reflecting surfaces, which may be of any desired vshape andof ary desired curvature and of different strength of reflection. s

Figs. 3 and 4 show an arrangement in which the subsidiary cell 2 onwhich the light is projected by the mirror 'I is mounted behind the`main photo-cell I. This mirror or reflecting only in connection withexposure meters of the photo-cell type, but also in connection withallkinds of semi-automatic and completely automatic devices of thephoto-cell type that? the apparatus according to the invention can beemployed. Thus, for instance, also in connection'with the devicesdescribed in the British Patents Nos. 395,808 and 483,775 and in theFrench Patent No. 725,313.

Most of the various practical arrangements are illustrated on thefigures enumeratedv below. In the arrangement according to Figs. l and2, I denotes the fundamental photo-cell and 2 the N/compensatingphoto-cell; for the sake of greater simplicity these will be called inthis specification the main and the subsidiary photo-cell,-respectively. l 'I'he device for the measurement of exposure or forlight control is operated by the main photo-cell I, and the operation ofAthe latter is degreased more or less by the subsidiary photocell It isa preferable. possible arrangement to mount both photo-cells on a singlebase-plate. 'I'heir surfaces may be situated in the same plane, or indifferent planes'. In case the main photo-cell is situated e. g. in avertical plane,

, mental or the surface of the subsidiary photo-cell may be.`

facing obliquely upwards. It is, however, also possible just as well toimagine it arranged in any other plane, particularly -if use is made ofcertain reflecting devices, It is also possible to employ more than onesubsidiary photo-cell.

similarly it is also possible to employ subsidiary photocells having aconvex or concave surface instead of a plane sur-face, and. similarly,the

reflecting surfaces employed may also be of any desired shape.

Accordingto the arrangement shown by way of example on Fig. 1 thesurface area of the main photo-cell I is substantially greater than thatof the subsidiary photo-cell 2. In front of the subsidiary photo-cell 2a light filter may also be provided'which will allow more or less lightto surface may also be of concaveshape or it may comprise a plurality ofreflecting surfaces arranged at different angles.

The various kinds of apparatus ofthe photocell type described in whatprecedes "possess a function based on such optical, electrical ormechanical principles to be described in detail below, which will ensurethat in the case of side or opposed light the time of exposure measuredby the main cell, and, correspondingly the shutter aperture of theobject -lens should be increased. The same effect may also be achievedby calibrating'the time of exposure measured by the maincell in the caseof side and opposed light at the figure increased to the necessarycorrect figure, orl correspondingly by adjusting the operation ofphotographic4 cameras equipped with semi-automatic or completelyautomatic devices in a suitable manner, so that if the-source of lightAis behind the operator, this light coming from behind should beprojected on the fundamain vcell itself, or possibly on a specialsubsidiary cell, rand the exposure meter should in conformity with thegreater effect .of illumination of the said light measure a shorter timeof exposure, or in the case of photographic cameras equipped with thedevices in question adjusts smaller aperture of the object lens shutter.

In the case of this arrangement the photographer should take care toensure that the source of light behind him should not be covered duringthe handling of the apparatus by any part ofthe apparatus. Various kindsof circuit arrangements may be employed for the devices described above.The

most simple method is to connect the two photol cells directly into acommon circuit with the measuring instrument. This can be performed intwo different ways: (l) The two cells are connected in series inopposition to each` other, i. e. the identical lelectrodes (e. g. thepositive electrodes) are connected with each other, whilst the `othertwo (in this casev the negative) electrodes and 2.

are connected to the terminals of the instrument. Such an arrangement isillustrated on Figs. 1

(2) The two cells are connected in parallel in opposition to each other,i. e. their opposite poles are connected with each other, and it is tothese po1es that the instrument is connected. Such an arrangement isshown on Figs. 3 and 4.

Fig. 5 represents an arrangement possessing the advantage oi requiring asingle-coil instrusubsidiary cell 2 as compared to that received bythemain cell l.

lAlthough Figs. 1 to 5 are all representing exceiving lightpredominantly from directions other posure meters, it is neverthelessdirectly possible that the pointers operated in these devices should actthe role of the pointer employed on a photographic camera equipped witha. semi-automatic device, as described e. g. in the Hungarian Patent No.109,132, and indeed also the role of a pointer .employed on aphotographic camera equipped with a completely automatic device, as,

for instance, that of the pointer 3 shown on Fig. 1

of the Hungarian Patent No. 120,431.

1 claim:

1. In a photo-electric exposure meter, a lpair of photo-electric cellscomprising a main cell and a subsidiary cell, the former of which has asurface area substantially greater than the surface area of the latter,the main cell being posithan the photographic subject and lying in aplane which is angularly disposed with respect to the rplane of thesurface of the main cell, indicating means operated by said cells, andan electric circuit including the two cells and the indicating means,said cells being so connected in the circuit that the current generatedin each cell is in opposed relation to the current generated in theother cell.

2. In a photo-electric exposure meter as claimed in claim 1, the'combination wherein said cells are arranged in series in said circuit.

3. InY a photo-electric exposure meter as claimed in claim 1, the4combination wherein said cells are arranged in parallel in saidcircuit.

4. In a photo-electric exposure meter as claimed in claim 1, thecombination wherein reflecting means are employed vfor directing thelight surrounding the photographic subject onto the subsidiary cell.

' 5. In a photo-electric exposure meter as claimed in claim 1, thecombination wherein the electrical circuit has a pair of branch circuitseach including one of said cells, and a resistance in each branchcircuit, one end of the resistances LszL RIszDoRFER.

